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Power supply project

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I pretend to build a power supply based on a 230v to 30v transformer, my power supply needs to have a 0v-30v variable output and also +15v, -15v and +5v fixed output, using preferably the LM117 to regulate the 0v-30v variable output. I will also need a second +5v output to connect a USB port and a +12v to have fans so the power supply stays cool always (the fan isn't that much of a priority but it would be great).
I really need this but got no idea how to make it, I'm very desperate.
If any nice person can help me with my problem that would just save me.
Hope to get a reply,
thank you.

Joe G

welcome to the fun world of electronics:)
A good beginning would maybe using a labtop power supply for the main power, and would be a safer start before tackling a line voltage;) they usually put out 18+ volts w/ quite a few amps.


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The first thing you should do is be more specific about your requirements. What is the max current requirement for each output voltage? 12 V at 1 A is relatively simple. 12 V at 200 A is very difficult.



Active Member
Does your 30V transformer has a centre tap (ie 15-0-15).

Getting 0-30 V from LM117 is possible but not sure it can go down to 0V. What is the current required from the variable supply?



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The datasheet of an LM117 in a TO3 metal case shows how to connect it. Your 30VAC transformer can drive a bridge rectifier and have the pulsing DC smoothed with a large filter capacitor to produce about +40V or -40V but not both.
The regulator IC will heat with about 32W if your heatsink and fan are perfect (impossible) and the chip is at its maximum allowed temperature (not recommended) then the maximum output current at 1.25V will be only 32w/38.75V= 826mA.
The datasheet shows how transistors can be added to increase the max output current. An LM317 costs less than an LM117 but is not available in the TO3 metal case.


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You didn't reall say what your power requirements are.

This http://www.ebay.com/itm/DC-Step-Dow...rrent-Voltmeter-Ammeter-Red-LED-/221392791812 module from ebay may satisfy some of your requirements.

I picked up an open frame +5, +15 and -15 V supply for like $10.00 USD 40 years ago. I just had to put it in a case.
My next supply was a 0-30 V (selectable) 10 A that I usually used at 12 V at the time. Again for $15.00. I had to fix it though. I got a manual only a few years ago.

There is a nice power supply made that's LM317 based which has a selector switch that selects the transformer tap and sets it to one of the normal fixed voltages. http://www.minute-man.com/acatalog/Multi_Voltage_Economical_Power_Supply.html

Meanwell makes some cheap power supplies.

Herschel Peeler

New Member
0 to 30 is not a problem. + and - 15 is no problem. From the same transformer is a problem. No common ground.
The -15 line is the same wire as the ground of the 30 V line.
And yes, how much current are you talking about?

Nonetheless, here is a rough draft to get started. A center-tapped transformer is assumed. 1 Amp is assumed on all outputs. DON'T USE BOTH SECTIONS AT THE SAME TIME. Grounding problem.
For the LM317, the HV version is required.


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If your transformer has only one secondary winding and that winding does not have any taps, then doing everything you list in post #1 can be done with half-wave rectifiers, 3 LM317's, and one LM337.


schmitt trigger

Well-Known Member
Building a linear power supply is a very good beginner's project.

But as AG has mentioned, thermal management is the most challenging part. Not difficult, but challenging.
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